Electronic components and photovoltaic panels based on organic materials are particularly sensitive to oxidation phenomena created by water and dioxygen. To be able to increase the lifetime of such elements, it is necessary to protect them with a film which is as little permeable as possible to oxidizing gases.
Permeability measurements are expressed in water vapor transmission rate (WVTR). Such measurements express the quantity of gas orthogonally crossing the film surface per day (g·m−2·d−1). The most impermeable or the best barrier films have a WVTR value in the order of 10−6 g·m−2·d−1.
To perform this type of measurement and referring to FIG. 1, a protective film or sample is placed in a permeation cell (1), comprising an upstream chamber (2) and a downstream chamber (3). More specifically, the sample (4) is held close to the bottom (5) of the upstream chamber to close an opening (6) connecting the two chambers. After being closed with a cover, the upstream chamber is filled with a target gas (7), for example, water vapor. The downstream chamber (3) has a detection device (not shown) capable of detecting the gas present in the downstream chamber (3), and accordingly having diffused from the upstream chamber (2) through the sample (4), arranged therein.
Permeation device examples are described in documents U.S. Pat. No. 7,624,621 and U.S. Pat. No. 8,388,742.
Due to the importance of the signal-to-noise ratio to perform high-sensitivity measurements, the phenomenon of contamination of the downstream chamber with the target gas on installation of the sample is critical. Such a contamination phenomenon essentially comprises the adsorption of gas by the walls of the downstream chamber. To obtain a quality measurement, the gas present in the downstream chamber should only result from the diffusion through the sample and be able to be distinguished from the noise originating from the desorption of the gases having contaminated the downstream chamber.
The installing of the sample in the downstream chamber (2) requires an opening of the permeation cell, and thus an exposure of the chambers to the atmosphere, as well as touchy and long manipulations to correctly close the opening (6) with the sample (4). The manipulation time may in particular be related to the steps of installing the sample and of tightening seals ensuring the tightness between the sample (4) and the bottom (5) of the downstream chamber. During this operation, as long as the upstream chamber (2) is not closed, the upstream and downstream chambers are contaminated by the atmosphere. To allow fine measurements of permeability to gas species present in the atmosphere, it is then necessary to purge with a neutral gas or to create a high vacuum in the downstream chamber containing the detection device before performing the measurement to decontaminate as much as possible the downstream chamber and thus decrease the noise corresponding to the measurement target gas. The purging or pump-out time enabling to obtain a given background noise is all the longer as the time of opening to the atmosphere to install the sample is long. Thereby, high-sensitivity permeability measurements for which the background noise should be low currently require a long decontamination time in the downstream chamber, which accordingly lengthens the total measurement time.
Even though the downstream chamber contamination would only slightly impact the measurement sensitivity, certain detection devices however require a high vacuum in the downstream chamber to be able to operate. This is particularly true when the detection device is a mass spectrometer for which it is necessary to reach a sufficiently high vacuum level to be able to use it.
Now, the downstream chamber purging or pump-out time is also strongly connected to the time of contamination of the downstream chamber by the atmosphere and accordingly impacts the total time of an experiment. Thus, if fast measurements are successively performed over a large number of samples, as in the case of the helium permeation detection device described in U.S. Pat. No. 7,624,621, the time necessary to drain the downstream chamber after each change of sample may become longer than the measurement time and considerably lengthen the total time of the experiment.
Another disadvantage of current permeation cells relates to the holding of the sample (4) at the level of the opening (6) connecting the two chambers (2) (3). Generally, the sample (4) is sealed on the opening (6) by a frame (8) screwed to the bottom of the upstream chamber, which presses the sample against said bottom (FIG. 1). The frame thus exerts mechanical stress on the sample, capable of deteriorating it, for example, by deforming its structure or by scratching its surface, particularly at the level of the contact with the seals (9). Thereby, permeation measurements are often considered as destructive for fragile films. Thereby, if the sample is measured a first time, and then taken out of the cell, a second measurement on the cell is a problem due to a serious doubt as to the damaging of the sample during the first measurement. Indeed, the positioning of the sample in the cell is generally not very accurate, so that its installing in a cell may be performed in a configuration which differs from that of the first measurement. The above-mentioned deteriorations may thus be present at the level of the surface exposed to the target gas during the second measurement, which alters the measurement.
Further, the manipulation of a sample always induces a doubt as to the preservation of its integrity. Indeed, for example, the above-mentioned protective films have a decreased thickness and the manipulation of such films is usually very delicate.
Thus, a sample measured a first time is not considered to date as having kept a sufficient integrity to be measured a second time. This strongly limits its study in different conditions (particularly climatic conditions).
Another disadvantage linked to permeation cells of the state of the art is the phenomenon of lateral permeation along axis x such as shown in FIG. 2, that is, in the sample plane. A permeation measurement theoretically comprises measuring the flow of a target gas orthogonally crossing the sample, that is, along axis z such as shown in FIG. 2. Theoretically, the frame (8) enables to accurately define the surface of the sample in contact with the target gas. In practice, for samples comprising layers of different permeability, the target gas (7) may laterally diffuse into the less permeable layers. This for example occurs for films comprising a plastic substrate, forming a weak barrier against gases, covered with a dense organic layer, forming a strong barrier against gases. The surface of the sample exposed to the target gas thus no longer corresponds to that defined by the frame. Thereby, the measurements may be altered if the lateral permeation is not negligible as compared with the orthogonal permeation.